This invention relates to a receiver used for multiplex signal transmission systems which transmit two independent signals by amplitude modulation and angle modulation of single carrier.
The receiver circuit technologies for the multiplex signal are well explained in the following two articles, for example:
(i) "AM stereo: five competing options" IEEE Spectrum, pp 24;14 31 June 1978, PA1 (ii) E. F. Close, A. L. Kelsch, R. D. Streeter "A proposed AM-PM Compatible AM Stereo System" IEEE Transaction on C.E., vol CE-23 No. 3, pp 405-408 August 1977.
The displeasing sound and the transient noise are frequently generated by FM receivers. But we could find no discussion on this problem for the reception of the multiplex signal in these prior articles.
To accomplish the objects of the present invention we use an intermediate frequency detector and a signal switching circuit in a signal demodulation circuit block.
FIG. 1 shows an example of a receiver circuit block diagram used for the multiplex signal transmission systems. Here an explanation is given of amplitude modulation by (L+R) and angle modulation by (L-R), where L is the left signal covering stereo signals and R is the right signal.
In FIG. 1, the received signals at an input terminal 1 are applied to a frequency mixer 2 along with the local signals generated by a local oscillator 3, and an output of this frequency mixer 2 is applied through an intermediate frequency filter 4 to a limiter 5 and an intermediate frequency amplifier 7. An output signal generated by the limiter 5 is applied to a demodulator 6 which demodulate the (L-R) signals from the angle modulated carriers. On the other hand, an output signal generated by the intermediate frequency amplifier 7 is applied to an envelope detector 8 which demodulate the (L+R) signals from the amplitude modulated carriers. The (L-R) and (L+R) signals generated by the demodulator 6 and the envelope detector 8 are applied to a matrix circuit 9, and L and R signals are obtained at output terminals 10 and 11 of the matrix circuit 9. These L and R signals are applied to a stereo two-channel amplifier (not shown) and drive two speakers (not shown).
In a receiver of such construction, complete stereo signals can be demodulated when the frequency difference between the carrier frequency at the input terminal 1 and the oscillating frequency of the local oscillator 3 matches correctly with a predetermined intermediate frequency. However, as it is also well known, a considerable level of noise is generated from the demodulator 6 for demodulation of the angle-modulated signal when the frequency difference is slightly deviated from the predetermined intermediate frequency (i.e. at detuning due to an insufficient manual tuning operation). Considerable noises are thus contained in the stereo L and R signals obtained at the output terminals 10 and 11. This kind of noise appears also as a transient noise at the fringe area of an exact tuning point during searching for reception of the multiplex signal by dial knob operation and causes an unpleasant feeling to the operator of the receiver.
In FM receivers generally on the market, this type of problem is solved by using a muting function which does not produce sound from the speakers at times other than exact tuning. However, in the subject of this invention, the premise is that at least one of the signals among the multiplex signals will be amplitude modulated signal. If we apply the muting function used by FM receivers to the multiplex signal transmission system used in AM receivers, it would cause an unpleasant dial knob operation feeling which would be different from that of previous AM receivers.
In order to eliminate the above-noted shortcomings, this invention detects whether or not the intermediate frequency is the normal predetermined frequency, and performs (1) demodulation of the stereo signal in the same way as FIG. 1 when the intermediate frequency is the normal predetermined frequency, and (2) demodulation of only the (L+R) signal obtained at the envelope detector for detecting only the amplitude modulation component when the intermediate frequency is not the normal predetermined frequency. By this, the above mentioned unpleasant feeling is eliminated.